Under the influence of an applied electrical field, temperature, and humidity, Ag (either in a metallic form, or as an alloy such as CuAg, AgPd, or SnAg) may be converted to Ag+ ions, migrate, and re-deposit to form a new Ag metallic area. In an electronic device, this new Ag metallic area often takes the form of dendrites or “whiskers”, which short circuit conducting Ag traces. Electromigration requires four conditions: a mobile metal; a voltage gradient; humidity or a continuous film of water; and soluble ions. Ag migration is a problem because Ag is anodically soluble and requires low activation energy to initiate the electromigration process.
Two trends in the electronic industry are making electromigration of Ag an even more severe problem: one is the reduction in lead (Pb) in solder; the second is the continual miniaturization of circuitry. The Pb replacement solders contain increasing amounts of Ag metal, hence the greater opportunity for Ag migration. The reduction in circuitry dimensions means smaller separation between circuit elements resulting in higher electrical field gradients (Volts/m) driving the electromigration under both AC and DC conditions and increasing the potential for short circuits.
It is known that a soluble ionic species is essential to provide the ionically conducting pathway for the Ag migration to occur, and the nature of the ionic contaminants has a major impact on the severity of Ag migration. High ionic mobility is influenced by such factors as ionic radius, electro-negativity, electron affinity, and charge to size. The source of these ions is varied, but can usually be traced to the substrate, or residues of the circuitry fabrication process. High polarity and hydrophilic substrates such as glass and some epoxy substrates are particularly susceptible to Ag migration especially when exposed to high humidity and temperature.